IQ modulators are well known in the field of RF and microwave communications, finding use in both analog and digital modulation formats. IQ modulation is a method of modulating a carrier wave, which is typically but not always sinusoidal, with two baseband input signals. The two signals are oftentimes referred to as I (in-channel) and Q (quadrature-phase) components.
FIG. 1 is a block diagram of an example conventional I-Q modulator 5. It contains a local oscillator (or “LO”) 10 producing sinusoidal signals at a carrier frequency (designated as ωc where). The LO has two outputs, which are of equal magnitude and differ in phase by exactly 90 degrees. The signal from the LO 10 are multiplied in mixers 12, 14 by two independent baseband inputs, the I and Q inputs. These products of the I and Q inputs and the carrier frequency w, are summed to yield the frequency-converted result. Baseband inputs may contain any arbitrary waveforms, although the bandwidth of these is usually less than the carrier frequency.
Although ideal modulators will generate I and Q channels that have exactly the same amplitude gain across the desired frequencies, and will be out of phase from one another by exactly 90 degrees, real-world implementations of the I and Q signals do not have identical magnitudes and do not differ in phase by exactly 90 degrees. Additional non-ideal aspects of the I-Q modulator such as differing gains and phases between the two mixers can also be modeled as amplitude and phase imbalances between the I and Q LO signals. These imbalances may affect the quality of the generated signal from the modulator.
IQ imbalance degrades the quality of digital communications signals. Quality measurements such as sideband suppression ratio and Error Vector Magnitude (EVM) are made worse by the presence of these errors. The presence of IQ imbalance also reduces the tolerance for other impairments such as noise and distortion in the overall system.
In modern digital communications systems and test equipment, if these IQ imbalance errors are known or can be measured, then the signals driving the I and Q channels can be corrected, often called “pre-distortion,” using well known techniques to improve the overall output signal quality.
There are conventional methods for measuring the IQ imbalance, but these measurement methods suffer from deficiencies such as having inherently large measurement error, being tedious to measure, and testing methods suffering from being tested using signals not often found when the modulators are put in use. Thus conventional IQ imbalance testing methods are inaccurate, take too long, or are not applicable to production devices.
Embodiments of the invention address these and other limitations of the prior art.